Semiconductor lasers have gained influence in high power laser applications because of their higher efficiency, advantages in Size, Weight And Power (SWAP) and their lower cost over other forms of high power lasers. Many laser applications such as industrial cutting and welding, Laser Detection and Ranging (LADAR), medical engineering, aviation defense, optically pumping rare earth doped fiber lasers, optically pumping solid state crystals in Diode Pumped Solid State lasers (DPSS), fiber-optic communication, and fusion research, among others, require a high power and high frequency response. Due to their high power array outputs, edge-emitting semiconductor lasers are widely used in such applications. However, degradation of these edge-emitting lasers is common, primarily as a result of Catastrophic Optical Damage (COD) that occurs due to high optical power density at the exposed emission facet.
Vertical-Cavity Surface-Emitting Lasers (VCSELs), in comparison, are not subject to COD because the gain region is embedded in the epitaxial structure and is therefore not exposed to the outside environment. Also, the optical waveguide associated with the edge-emitter junction has a relatively small area, resulting in significantly higher power densities compared to VCSELs. The practical result is that VCSELs can have lower failure rates than typical edge-emitting lasers.
To date, VCSELs have been more commonly used in data and telecommunications applications, which require higher frequency modulation, but not as much power. VCSELs have offered advantages over edge-emitting LASERs in this type of application, including ease of manufacture, higher reliability, and better high frequency modulation characteristics. Arrays of VCSELs can also be manufactured much more cost efficiently than edge-emitting laser arrays. However, with existing VCSEL designs, as the area of the array grows the frequency response has been penalized by heating complexities arising from the multi-element designs, parasitic impedances, and the frequency response of the wire bonds or leads required by the high current. Thus, the modulation frequency of the array decreases.
VCSELs and methods for manufacturing them are known. See, for example, U.S. Pat. Nos. 5,359,618 and 5,164,949, which are incorporated herein by reference. Forming VCSELs into two-dimensional arrays for data displays is also known. See U.S. Pat. Nos. 5,325,386 and 5,073,041, which are incorporated herein by reference. Flip-chip multibeam VCSEL arrays for higher output power have been mentioned, in particular, in U.S. Pat. No. 5,812,571, which are incorporated herein by reference.
However, VCSEL arrays that provide both high frequency modulation and high power have not been adequately developed. Furthermore, arraying such devices together increases heat generation, adding to the negative effects on high frequency operation.
In addition, free space optical links that are intended for short range mobile device communication are generally designed with optical elements for efficient transmission of a low divergence beam (using a collimating optic) and for efficient reception of the incident light (using a collecting lens). Since high speed detectors are very small, around 60 μm in diameter for 5-10 Gb/s speeds, the collecting optic has to focus the light down to a small spot to get a good signal-to-noise ratio. Such systems are therefore very alignment sensitive, as the small spot can easily miss the small detector if something moves or perturbs the alignment. This has made free space optical communication between mobile devices difficult. The exception has been the IrDA (Infrared Data Association) standard which uses LED-based transmission into a very broad transmission beam and a hemispherical collection optic. While free space optical links were popular for a while at relatively low speeds, as mobile device concepts have evolved, a need has arisen for high bandwidth communication between two devices that can actually be touching each other or separated by just a few millimeters. There are radio frequency approaches that will work in these near field ranges, but they have disadvantages, including omnidirectional transmission which is a security concern and regulatory issues due to RF interference concerns.